Despite promising property combinations such as high hardness, tensile stress and fracture strength, practical applications of metallic glasses and nanomaterials have been relatively limited. One issue that has arisen in both material classes is that the materials may exhibit relatively brittle response. Commercial exploitation of these material classes has been facilitated by utilizing their soft and hard magnetic properties for applications including transformers and high energy density permanent magnets and, more recently, for surface technology applications whereby coatings including these materials may be applied to a surface to solve corrosion, erosion, and/or wear issues.
Although both metallic glasses and nanomaterials can show ductility when tested in compression, the same materials when tested in tension, may generally exhibit a tensile ductility which may be close to zero and fracture in a brittle manner. Due to the extremely fine length scale of the structural order (i.e. molecular associations) and near defect free nature of these materials (i.e. no 1-d dislocation or 2-d grain/phase boundary defects), relatively high strength may be obtained. However, due to the lack of crystallinity, dislocations may not be found and so far there does not appear to be a mechanism for significant (i.e. >2%) tensile elongation. Metallic glasses may exhibit relatively limited fracture toughness associated with the rapid propagation of shear bands and/or cracks which may be a concern for the technological utilization of these materials.
In metallic glasses deformed at room temperature, plastic deformation may be inhomogeneous with cooperative atomic reorganization in shear transformation zones, which may take place in thin bands of shear bands. In unconstrained loading such as under tension, shear bands may propagate in a runaway fashion followed by the commensurate nucleation of cracks, which may result in catastrophic failure. For nanocrystalline materials, as the grain size is progressively decreased, the formation of dislocation pile-ups may become more difficult and their movement may be limited by the large amount of 2-d defect phases and grain boundaries. Reductions in grain/phase size may render otherwise mobile dislocations immobile due to the effective disruption of slip systems in the grain/phase boundary area. As a result, the ability of nanoscale materials to exhibit significant levels of plastic deformation may be suppressed even in very ductile nanoscale FCC metals such as copper and nickel. Thus, the achievement of adequate ductility (>1%) in nanocrystalline materials has been a challenge. The inherent inability of these classes of material to be able to deform in tension at room temperature may be a relatively limiting factor for potential structural applications where intrinsic ductility may be needed to avoid catastrophic failure.